Executive Summary
Relevance to the DOE Wind Power Program and wind industryTall onshore wind turbines, with hub heights between 80 m and 100 m, can extract large amounts of energy from the atmosphere since they generally encounter higher wind speeds, but they face challenges given the complexity of boundary layer flows. This complexity of the lowest layers of the atmosphere, where wind turbines reside, has made conventional modeling efforts less than ideal. To meet the nation's goal of increasing wind power into the U.S. electrical grid, the accuracy of wind power forecasts must be improved.In this report, the Lawrence Livermore National Laboratory, in collaboration with the University of Colorado at Boulder, University of California at Berkeley, and Colorado School of Mines, evaluates innovative approaches to forecasting sudden changes in wind speed or "ramping events" at an onshore, multimegawatt wind farm. The forecast simulations are compared to observations of wind speed and direction from tall meteorological towers and a remote-sensing Sound Detection and Ranging (SODAR) instrument.Ramping events, i.e., sudden increases or decreases in wind speed and hence, power generated by a turbine, are especially problematic for wind farm operators. Sudden changes in wind speed or direction can lead to large power generation differences across a wind farm and are very difficult to predict with current forecasting tools. Here, we quantify the ability of three models, mesoscale WRF, WRF-LES, and PF.WRF, which vary in sophistication and required user expertise, to predict three ramping events at a North American wind farm.A brief look at the forecast models WRF mesoscale: In Chapter 2, the numerical Weather and Research Forecasting model (WRF) is applied to the wind park domain in mesoscale forecasting mode, with no perturbations made to the model physics. Here, WRF is considered the control run and is used to simulate an "out-of-thebox" version of WRF which requires minimal expertise from the user and minimal input about site conditions. Mesoscale WRF with "skilled" user input changes to horizontal and vertical resolution, nesting levels, and land surface conditions is applied in Chapter 3.
WRF-LES:A novel application of large-eddy simulation (LES) to wind energy forecasting is examined in Chapter 3. WRF-LES is a high-resolution, turbulence-resolving mode of WRF. Here, WRF-LES is tested with two LES closure schemes: a turbulent kinetic energy closure of order 1.5 (TKE 1.5) and the Dynamic Reconstruction Model (DRM).
PF.WRF:The influence of subsurface hydrology and hence, surface exchanges of heat and moisture on wind conditions is examined in Chapter 4. PF.WRF is a fully-coupled, groundwater-to-atmosphere model which incorporates the 3-D, variably-saturated hydrologic model ParFlow and WRF to simulate physics from bedrock to the top of the atmosphere.
A brief look at the forecasted ramping eventsThe ramping events include times of rapid increases in wind speed during a synoptically-forced period in February (Case S), a ...